Most revealed only small traces of horse in the meat content – up to 0.3 per cent. However in one sample, from Tesco Everyday Value Beef Burgers, horse meat accounted for around 29 per cent of the meat content.

The affected burgers were produced by two processing plants, Liffey Meats and Silvercrest Foods in Ireland, and one British plant, Dalepak Hambleton, in North Yorkshire.

Professor Alan Reilly, chief executive of the FSAI, emphasised that the findings posed no risk to public health but said there was no reasonable explanation for the findings.

He said: “In Ireland it is not in our culture to eat horse meat and therefore, we do not expect to find it in a burger. Likewise, for some religious groups or people who abstain from eating pig meat, the presence of traces of pig DNA is unacceptable.”

Tesco last night apologised for any distress and announced they would be withdrawing both affected products as well as all brands from the supplier as a “precaution”.

A spokesman said: “We are working with the authorities in Ireland and the UK, and with the supplier concerned, to urgently understand how this has happened and how to ensure it does not happen again. We will not take any products from this site until the conclusion and satisfactory resolution of an investigation. We will not tolerate any compromise in the quality of the food we sell.”

An Iceland spokesman said: “Iceland has withdrawn from sale the two Iceland brand quarter pounder burger lines implicated in the study.”

Aldi removed their only affected product from sale and were conducting their own investigation.

Lidl also removed all of their three products from sale pending a full investigation.